1. Field of the Invention
This invention relates to recording holograms and, more particularly, to methods and techniques to improve the quality, recording speed, efficiency and color displaying of holograms. These methods may find particular applications with photorefractive polymers or other holographic recording materials for integral holographic 3D display and other applications.
2. Description of the Related Art
Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that the object appears as if the object is in the same position relative to the recording medium as it was when recorded. Alternately, holograms can be computer generated by calculating the modulation pattern that would have been formed if beams with certain characteristics (wave front, intensity) had crossed each other. The calculated pattern is next transferred to a medium to make the actual hologram. The image changes as the position and orientation of the viewing system change in exactly the same way is if the object were still present, thus making the recorded image (hologram) appears three-dimensional. The technique of holography can also be used to optically store, retrieve, and process information (data storage); or to generate a particular wavefront (holographic optical element “HOE”). An HOE is an optical lens or lenses that converts an incoming wavefront to another wavefront using the diffraction principle. While holography is commonly used to display static 3-D pictures and simple 3D images have been successfully rendered, the generation of arbitrary scenes by a holographic display is not yet possible.
As shown in FIG. 1, when two coherent optical beams e.g. a reference beam 10 and an object beam 12 of finite beam width cross each other at a point in space, they interfere. The phase difference between the beams at each spatial location in the plane defined by the bisector of the vectors formed by the two beams defines the intensity pattern. The recording of this intensity variation into a medium 14 as a phase and/or intensity modulation results in the formation of a hologram 16. The object beam may be scattered from the object or modulated based on computer-generated images to produce a 3D image. Typical media include, photothermoplastics, photopolymers, photochromics, silver halide, plates or films, and photorefractive (PR) polymers and crystals.
There exist two types of geometries for recording holograms, transmission and reflection. The geometry strongly influences the properties of the hologram and the way the hologram should be replayed. FIG. 1 illustrates the so-called transmission geometry where the recording beams 12 and 10 are incident on the same side of the holographic storing medium 14. In FIG. 2, recording beams 50 and 52 are incident from opposite sides of the holographic storing medium 54. This is the reflection geometry.
As shown in FIG. 3, when a light beam 60 is directed at the appropriate incident angle to read a transmission hologram 66, the diffracted beam 62 emerges from the opposite side of the material 64. In the case reading beam 60 contains different wavelengths, the diffracted beams 62 and 68 exit the media 64 at different angles and with different diffraction efficiency according to the hologram characteristics.
As shown in FIG. 4, when a light beam 70 is directed at the appropriate incident angle to read a reflection hologram 76, the diffracted beam 72 is diffracted to the same side of the material 74. In the case reading beam 70 contains different wavelengths, the diffracted beams 72 and 78 exit the media 74 at different angles. Moreover, if the reading beam 70 has a different wavelength from the writing beam used to record the hologram, the diffracted intensity is strongly attenuated (Bragg regime hologram). This is referred to as color selectivity.
Photorefractive Polymer Material
PR polymers are dynamic holographic recording materials. As shown in FIG. 5, an embodiment of a recording device 20 includes a layer of PR polymer material 22 sandwiched between a ground electrode 24 on an electrode support 26 and a high-voltage electrode 28 on an electrode support 30. A high-voltage power supply 32 applies a voltage between ground electrode 24 and high-voltage electrode 28 to provide a bias electric field 34 across the PR polymer.
As shown in FIG. 6, in PR polymers, a three-dimensional refractive index pattern—a phase hologram 40 replicates the non-uniform interference pattern 42 formed by the two incident coherent light fields 44 and 46. This effect is based on the build-up of an internal space-charge field Es 48 due to selective transport and trapping of the photo-generated charges 50, and an electric field induced index change via the electro-optic effect. This process—in contrast to photochemical processes involved in photopolymer holograms—is fully reversible, as trapped charges can be de-trapped by uniform illumination. The erasability of the PR gratings allows for refreshing/updating of the holograms. In a typical PR material the holograms are viewed with the help of a reading beam, as long as the initial writing (recording) beams are present. When the writing beams are turned off, the hologram written in the PR polymer decays at a rate determined by the material properties and ambient temperature.
The quality of the PR polymer and the written hologram can be evaluated with respect to three parameters: sensitivity, diffraction efficiency and persistence. The sensitivity is usually defined as the first temporal derivative of the dynamic efficiency measurement at the origin divided by the writing beams power. However, it is sometime more convenient to use the notion of writing time. Writing time and sensitivity are related quantities. As shown in FIG. 7, in continuous wave illumination, the writing time Tw is simply how long the writing beams must illuminate the media to achieve the desired diffraction efficiency of the hologram. The diffraction efficiency 82 is defined as the ratio between the intensity of the diffracted beam and the incident beam. In theory, PR polymers can achieve up to 100% diffraction efficiency. The persistence is a measure of the rate of decay of the hologram.
Holographic 3D Displays
Holographic 3D displays are one application of re-writable holograms. Computer generated holographic 3D displays provide highly realistic images without the need for special eyewear, making them valuable tools for applications that require “situational awareness” such as medical, industrial, and military imaging. Current commercially available holographic 3D displays employ material like photopolymers or silver halide emulsion that lack image-updating capability, resulting in their restricted use and high cost. Dynamic updateable 3D holographic displays based on acousto-optic, liquid crystal display and MEMS based recording media have been demonstrated. Unfortunately, these devices do not have memory, and thus do not exhibit persistence of recorded images. The lack of persistence results in the requirement of update rates faster than 30 Hz to avoid image flicker. Since 3D images exhibit very high information content, this high refresh rate requirement currently limits real time holographic displays to small sizes or low resolution. Although updateable, PR polymers have not yet been used for 3D displays because of their long writing times and low persistence. The ultimate goal is to be able to write at high enough rates to provide near video capability. An interim goal is provide the capability to update holograms with reasonable write times, high diffraction efficiency and persistence long enough to view the display. To extend dynamic holographic 3D displays towards practical applications, alternative materials with high efficiency, reversible recording capabilities, memory, and significantly larger sizes are needed.
Integral Imaging
Integral imaging (also called integral photography) is an auto stereoscopic technique that allows reproduction of 3D images with a flat (2D) display. Auto stereoscopic means there is no need for the viewer to wear eyewear to see the 3D effect. This technique is summarized in FIG. 8 where an array of lenses 92 is placed in front of the regular flat display 90 (static or dynamic) to redirect the light coming from individual pixels 98 so the viewer see different information at different angles. Individual lenses 96 of the lens array cover a defined surface of the display 94. The vertical and horizontal resolution of this kind of 3D display is defined by the number of lenses per unit of surface. The angular resolution is defined by the number of individual pixels covered by an individual lens 96. There exist 3D cards and 3D television systems based on this principle.
It has to be noted that horizontal parallax only (HPO) system can be achieved if the spherical lens array 92 is replaced by a cylindrical lens array. In this case, the 3D effect is only seen in one dimension (horizontal if cylindrical lenses axis is vertical). HPO is interesting since it did not degrade the vertical resolution of the display 90, and reduces the number of individual lenses 96. Since human vision relies mainly on the horizontal separation of both eyes to determine depth, vertical parallax can be ignored without degrading the 3D information too much.
Integral Holography
Holography can also be used for integral imaging (referred to as “integral holography”). In this case, what was a collection of individual pixels 94 is shrunk into a single holographic pixel (hogel) by a lens. The function of the individual lenses 96 that collimate the beams is also recorded as a hologram into the material. The advantage is that the resolution can be much higher than with a regular lens array and 2D display since the pixels and lens do not need any physical embodiment. The resolution limitation now comes from the optical recording setup resolution that is usually orders of magnitude higher than regular lens array resolution.
A typical integral holographic optical system using a grey-scale (single-color) transmission geometry is shown in FIG. 9. A laser 300 emits a coherent beam 310 that is split into a reference beam 320 and an object beam 330 by a beam splitter 340. The object beam is expanded by means of a telescope 350 and the beam's amplitude is structured by a device 360 that can be a transparent image, a mask, a spatial light modulator etc. The object beam is then resized by a telescope 370, directed by one or several mirrors 380, and focused by a lens 390 to the holographic recording material 400. That lens 390 is spherical in the case of full parallax and cylindrical in the case of horizontal parallax only. The reference beam 320 is shaped by optics 410 to match the shape of the object beam at the holographic material location, and directed by a mirror 420 to the holographic recording material. After one hogel has been recorded, the material is moved to the next hogel location by a translation stage 430. Controlling electronics 500 ensures the synchronization between the laser 300, the translation stage 430 and the device 360 that structures the object beam. In the case the device 360 that structures the object beam 330 is electronic; a memory 510 could be used to store the hogel data. During recording, the material is positioned or otherwise shielded so that the powerful writing beam is not incident on a viewer's eyes for safety.
When all the hogels have been recorded, the material is processed to develop the hologram (if needed) and moved to the reading position 440, where a reading source 450 emits a light beam 460 that is expanded by a telescope 470 and diffracted by the hologram 440 in a diffracted beam 480 toward the viewer's eyes.
In the case of a refreshable holographic recording material, when the hologram needs to be erased, the holographic recording device 400 is moved to the erasing location, where the erasing process occurs. In the case of erasure by light, like with photorefractive materials, an erasing light source 600 emits a beam 610 that is expanded with a telescope 620 and illuminates the whole hologram area. Some materials need a heating process for the hologram to be erased. Some materials like photo thermo plastic need electrical charging and heating the material to erase the hologram. Once erased, the recording material is moved back to the recording position.